For example, the conductive polymer is highly conductive, and therefore, it is used as a solid electrolyte of a solid electrolytic capacitor such as tantalum solid electrolytic capacitor, aluminum solid electrolytic capacitor, and niobium solid electrolytic capacitor.
For example, the conductive polymer used in this application can be obtained through chemical oxidation polymerization or electrolytic oxidation polymerization of thiophene or its derivatives.
In carrying out the chemical oxidation polymerization of thiophene or its derivatives above, an organic sulfonic acid can be usually used as a dopant. In particular, it is known that an aromatic sulfonic acid is suitable. As an oxidant, a transition metal is used. In particular, it is known that ferric compound is suitable. Usually, a ferric salt of an aromatic sulfonic acid can be used to serve as a dopant and oxidant in the chemical oxidation polymerization of thiophene or its derivatives.
Among the ferric salts of the aromatic sulfonic acid, following reports are there. Particularly useful are ferric toluenesulfonate and ferric methoxybenzene sulfonate. In order to synthesize a conductive polymer using them, they are used as a dopant and oxidant, while being mixed with a polymerizable monomer such as thiophene or its derivatives. These processes are simple and suitable for the industrialization (see patent reference No. 1, and patent reference No. 2, below).
However, when a conductive polymer was obtained by using ferric toluenesulfonate serving as an oxidant and dopant, it did not attain the characteristics fully satisfying the initial resistance value and the heat resistance value. Also, when a conductive polymer was obtained by using ferric methoxybenzene sulfonate as an oxidant and dopant, an improvement was found compared with the conductive polymer using the ferric toluenesulfonate, in that the initial resistance value was lowered and the heat resistance was improved; however, it still did not reach the characteristics that could be fully satisfied.
The reasons of the results above are because ferric toluenesulfonate and ferric methoxybenzene sulfonate are solids, so that they are usually used in a state of solution in an alcohol. However, the solution produces precipitations during storage.
That is, when using an alcohol solution of ferric toluenesulfonate or ferric methoxybenzene sulfonate, producing precipitations, uniformity can be deteriorated. Therefore, when solid electrolytic capacitors are prepared by using such conductive polymers, ESR (equivalent series resistance) can be increased, and the reliability under a hot condition can be decreased.
In addition, when thereby obtained conductive polymer is used as a solid electrolyte of a solid electrolytic capacitor, the following issues have to be considered. That is, a conductive polymer obtained by means of a chemical oxidation polymerization method is usually not soluble in a solvent, and therefore, it is necessary to form a layer of the direct conductive polymer directly on an element. The element here includes: a positive electrode of a porous body made of a valve metal such as aluminum, tantalum and niobium; and a dielectric layer made of an oxide film of that valve metal.
However, the formation of the conductive polymer directly on the element requires working under a condition that is very complicated. Therefore, its reproducibility was poor, and the process control was very difficult.
Considering the situations above, there has been variously considered soluble conductive polymers (see patent reference No. 3). Patent reference No. 3 reports that a dispersion liquid of a conductive polymer can be obtained from mixture and reaction from polystyrene sulfonic acid, ammonium persulfate, an iron salt, and ethylenedioxy thiophene. However, thereby obtained conductive polymer did not have enough conductivity, and therefore, further improvement of the conductivity is required in order to use it as a solid electrolyte of a solid electrolytic capacitor.